A control device of a continuously variable transmission for a vehicle (hereinafter, continuously variable transmission) is well known that has a pair of variable pulleys with variable effective diameters of an input-side variable pulley (a primary pulley, a primary sheave) and an output-side variable pulley (a secondary pulley, a secondary sheave) and a transmission belt wrapped around between the pair of the variable pulleys and that respectively controls an input-side thrust force (a primary thrust force) in the primary sheave and an output-side thrust force (secondary thrust force) in the secondary sheave to set an actual gear ratio to a target gear ratio and preventing a slip of the transmission belt. This corresponds to control devices for vehicles described in Patent Documents 1 and 2. Generally, in such a continuously variable transmission, for example, the primary thrust force and the secondary thrust force are set to control a pulley pressure (primary pressure) in the input-side variable pulley and a pulley pressure (secondary pressure) in the output-side variable pulley such that a target gear ratio is achieved while preventing a belt slip, and feedback control is provided based on a deviation between an actual gear ratio calculated by using detection values of an input-side rotation speed and an output-side rotation speed in the continuously variable transmission (actual gear ratio=input-side rotation speed/output-side rotation speed) and a target gear ratio to fine-tune and control the primary pressure (or primary thrust force), for example.
To accurately calculate the actual gear ratio, the input-side rotation speed and the output-side rotation speed must more accurately be detected by a rotation sensor, for example. However, because of the characteristics of the rotation sensor, for example, a rotation speed may not accurately be detected in an extremely low rotation speed range and a detection value of a rotation speed may not reflect an actual rotation speed (actual rotation speed). If such a rotation sensor is used, the target gear ratio can properly be achieved in the running state in which a detection value of a rotation speed reflects the actual rotation speed. However, in the running state in which a detection value of a rotation speed does not reflect the actual rotation speed, the actual gear ratio is not known and the target gear ratio may not properly be achieved because of control variations etc. in a thrust force (or pulley pressure) and an input torque assumed in this case. Particularly, to ensure the vehicle restart during vehicle stop or the reacceleration performance during low-speed running, it is desired to maintain a gear ratio of the continuously variable transmission at a lowest-speed-side gear ratio (a Lowest, a maximum gear ratio). Therefore, during vehicle decelerated running while a gear ratio is controlled toward the lowest-speed-side gear ratio, it is desirable to control the gear ratio to the lowest-speed-side gear ratio during the running state in which a detection value of a rotation speed from the rotation sensor reflects the actual rotation speed.
However, for example, during rapid deceleration running, the gear ratio may not be returned to the lowest-speed-side gear ratio during the running state in which a detection value of a rotation speed reflects the actual rotation speed and it may become unknown whether the gear ratio is finally set to the lowest-speed-side gear ratio. For such a problem, for example, Patent Document1 proposes to fix the pulley pressures (primary pressure and secondary pressure) and limit the input torque at the restart of a vehicle in accordance with the determination of possibility of an improper speed change preventing a gear ratio from returning to the lowest-speed-side gear ratio when a vehicle is stopped, and to cancel the fixation and limitation when a rotation sensor can properly detect a rotation speed.